" The terrestrial stock consists of two kinds of resources: those renewable on a human time scale and those renewable only over geologic time and which, for human purposes, must be treated as nonrenewable. Terrestrial low-entropy stocks may also be classified into energy and material. Both sources, the terrestrial and the solar, are limited. Terrestrial nonrenewables are limited in total amount available. Terrestrial renewables are also limited in total amount available and, if exploited to exhaustion, become just like nonrenewables. If exploited on a sustained-yield basis, then they are limited in rate of use, though practically unlimited in terms of the total amount eventually harvestable over time. Likewise, the solar source is practically unlimited in total amount but strictly limited in its rate and pattern of arrival to earth. Thus both sources of low entropy are limited. "

― Herman E. Daly , Steady-State Economics